Dozens of variable radius transmissions have been invented for bicycles, none of which has experienced commercial success. The reason so many people invent variable radius transmissions is because they are more convenient to use than the derailleur drives which currently dominate the bicycle market. Specifically, variable radius transmissions can change speeds under power, and they do so very smoothly, whereas the rider of a derailleur drive must ease up on the pedals while shifting, and even then the shifting operation is not always smooth. Also, the gear ratios provided by variable radius transmissions cascade evenly from the highest to lowest speed, whereas the gear ratios of the typical 10-speed derailleur drive jump about erratically. For example, on the typical 10-speed bicycle, sixth speed has a gear ratio between that of second and third.
Unfortunately, all variable radius transmissions invented to date suffer from disadvantages that outweigh their advantages. Specifically, they are too heavy, too bulky, too difficult to perform maintenance on, too expensive, and/or too fragile. The 10-speed derailleur drive is not very convenient to operate, but it is inexpensive and can handle dirt, mud, water, and neglect better than every other alternative transmission ever invented.
In order for a variable radius transmission to appeal to consumers it must contain few and inexpensive parts, it must be easy to maintain, and it must be rugged enough to survive road dirt, rain, and neglect. However, the typical design for variable radius transmissions does not seem capable of meeting these requirements. This design, of which there are dozens of variations, includes a disc with a plurality of radial slots. A slidable drive segment is mounted in each slot. The segments are coupled together in some manner and can be slided radially in and out in unison. Unfortunately, this type of variable radius transmission is always more complex than a derailleur drive, which in turn makes it more expensive and more difficult to maintain. It also has a shorter life because its components are subjected to high stress and friction; the reason being that the segments are forced to slide while engaged to the chain and, therefore, under heavy load. Furthermore, this type of variable radius transmission requires liquid lubricants to permit the segments to slide, but liquid lubricants attract dust and grit, which abrade the components. The dirt could also build up to the point at which it clogs the mechanism and renders it useless.
In an attempt to rectify those problems, three variable radius transmissions were patented in the 1970's in which the drive segments are independent instead of coupled together. These inventions described in U.S. Pat. Nos. 3,798,989, 3,861,227, and 4,030,373. The segments in these inventions slide only when disengaged from the drive belt; therefore, they do not experience as much stress and abrasion when they slide. Each drive segment in these inventions contains a spring biased retaining means to lock it at a particular position. The effective radius of these inventions is controlled by a gate which is moveable radially. The drive segments must pass through this gate, and when the gate is moved to a different radial position the segments collide with it and are pushed to a different position.
Unfortunately, the first mentioned of the inventions suffers from the inability to provide more than three speeds. Also, its effective shape varies from a small circle to a large oval, instead of from a small circle to increasingly larger circles.
The second of the previously mentioned inventions actually made it to the point of test marketing, but it was soon abandoned because of its short lifetime and noisey operation.
The last mentioned invention has not been put out on the market, and if it was, consumers would certainly dislike its complexity. Furthermore, its use of V-belts, a friction plate, screws, and tiny universal joints make the invention very susceptible to mud, dirt, rain, and fog. Indeed, it is considered very poor engineering practice to use friction drives in slow speed, high torque applications because they tend to slip. Also, the drive segments in this invention are held at specific radial locations by gripping tiny serations in their tracks, but if the segments require liquid lubricants then the serations will quickly clog with dirt. Because this invention is so easily harmed by dirt and moisture, the patent calls for a protective cover to completely enclose the entire drive train, thereby shielding it from the outside world. Unfortunately, protective covers have been available for bicycle gearing systems for decades as a method to protect the chain from dirt and water, but most consumers refuse to use them because of their inconvenience. Therefore, in order for a bicycle gearing system to appeal to consumers it must be rugged enough to operate exposed to airborne dirt and moisture, and ideally it would also be unaffected by the mud that is kicked up by the tires. Consumers have so far been willing to buy only a partial cover, so that is all a bicycle gearing system should require.
It is an object of my invention to provide a variable radius gearing system with all the characteristic advantages of such transmissions, as previously mentioned, but one which contains few and inexpensive parts, is easy to maintain, and is so rugged it is unaffected by airborne dirt and water.